It is (explicitly, as a planned-and-tested-for-outcome) to cause players to not automatically attach "$3.50" to an in-game purchase which costs "3 dragon egs", thus not activating as much resistance to purchasing as it would if the prices were trivially calculable. (For instance, if you sold 10 dragon eggs for $10.)
You want to make it very obvious to people what they're paying for eggs, but very, very, very easy to spend eggs once they have them, both because a) it's in your best interest that they run out of eggs and re-buy them and b) because you will not be able to recognize revenues for eggs which are bought but not converted into in-game assets. (Revenue recognition for this is a deep and weird topic. Ask me some other time.)